1. Technical Field
The invention relates to abrasive products of the type having an active part consisting of a "compact" containing diamond grains or particles in an amount of more than 80% by volume o the compact, each grain being directly bonded to its neighbours so as to form a polycrystalline structure. It further relates to compact type products consisting of a polycrystalline structure and of a hard and refractory support integral with the compact.
The work "compact" designates a sintered product formed by grains bonded together by bridges created by diffusion of material in plastic state. Such plastic phase sintering is obtained, with diamond grains, at pressures and temperatures of the same order of magnitude as the pressures and temperatures used for synthesis of the same grains, which may be reduced by adding well-known conversion catalysts to the starting products.
2. Prior Art
Products of the above-defined type are known (U.S. Pat. No. 3,239,921) which are obtained at a temperature which may exceed 1750.degree. C., in which the voids of the compact are filled by conversion catalyst, such as Co, Va, Ti, Zr, Cr, Si. Products are also known in which a compact is directly bonded to a metal carbide support (tungsten carbide in general). These products have the drawback of being rapidly degraded either when sintering was carried out without a sufficient amount of diamond particles, or when the active part is heated to a temperature exceeding about 700.degree. C., due to the stresses induced in the metal matrix by thermal expansion of the matrix and due to the tendency of the diamond in contact with catalyst to graphitise when heated to a high temperature without at the same time being subjected to a high pressure.
The resistance of the compact at high temperature may be improved by removing the catalyst using the leaching method described in U.S. Pat. No. 3,745,623 (Wentorf) but the residual compact is porous and has a very large spedific surface, so that the life duration of the compact is reduced due to oxidation when it is brought to a high temperature.
Abrasive products have also been proposed including a silicon carbide support and a polycrystaline diamond compact, which is not in sintered condition since it was not subjected during manufacture to pressures and temperatures sufficient to cause mutual intergrowth of the diamond particles, in which the voids between grains of the composite contain a compound of silicon and a metal such as nickel (U.S. Pat. No. 4,241,135 to Lee et al). A drawback of such products is insufficient abrasion resistance, due to the lack of sintering.
A composite abrasive product has further been proposed (U.S. Pat. No. 4,124,401) which comprises: a mass of polycrystalline diamond cemented by a siliconcontaining binder; and a carbide support whose cohesion is obtained with cobalt. The lack of catalyst and sintering during manufacture of the diamond mass prevents the formation of direct bridges between the diamond grains. What is obtained is not a sintered compact having a skeleton of great rigidity but rather a product which may be regarded as cemented by the binder.
U.S. Pat. No. 4,380,471 describes abrasive products comprising a cobalt-bonded tungsten carbide substrate, a barrier preventing migration of cobalt into diamond and a polycrystalline diamond composite having a silicon-Rhenium binder. Sintering of that product is not complete although the components are subjected to high temperatures and pressures during manufacture since they are not applied during a time sufficient for the binding phase to play a function other than filling the voids. Consequently, the product isnot adapted to resist abrasion.
The same comments are applicable to compacts manufactured by directly sintering diamond particles with silicon and nickel only (European No. 116,403). The failure to adequately support abrasion is probably caused by an insufficient "wettability" of the catalyst which is used. Wettability of the catalyst is a parameter which directly affects the reactivity of the latter in respect of the diamond particles. Insufficient wettability may be due to a melting point which is not low enough. Then, under the usual sintering temperature and pressure conditions, bridging occurs to an extent which isnot sufficient for the body to resist during machining. Another drawback of the last-mentioned compact is that, although it is sintered and it is thermostable, it cannot be brazed due to the lack of support and consequently its field of use is restricted. A number of abrasive compacts obtained by sintering diamond particles in presence of a hard thermostable substance which is not diamond have also been proposed. Among such substances, as defined for instance in French 2,043,350, boron, titanium, silicium and their compounds have been used and represent a substantial improvement. However, bridging often remains insufficient for a satisfactory resistance to shocks and abrasion.
From the above review of prior art products, it appears tha there is no existing abrasive compact or composite, with or without a support, which exhibits all thermostability, abrasion resistance and shock resistance, which are now required from abrasive products.
It is an object of the invention to provide a thermostable diamond abrasive product which is improved as comared with the existing products, particularly in that it includes a compact in which diamond grains are directly bonded by bridbes, which has a low specific surface, is resistant to shock and abrasion and can be brazed.
For that purpose, there is provided a product of the above-defined type, wherein the porosity or voids between diamond grains are filled with a binder phase which contains, on the one hand, silicon or titanium and, on the other hand, nickel (if titanium is used) or iron-alloyed nickel. Silicon or titanium forms a hard thermostable ceramic phase at the surface of the diamond particles.
In an advantageous embodiment, the compact is bonded by a metallurgical interface to a refractory metal substrate.
The diamond grains represent 80% to 95% by volume of the compact, advantageously 80 to 90%. The mutual proportions by weight of titanium and Fe-Ni in the binder phase are advantageously in a ratio between 40/60 and 90/10.
The substrate of refractory metal (typically tungsten or molybdenum) may be carried by a tungsten carbide or ceramic material support. A diffusion barrier may be placed between the metal substrate and the compact so as to accomodate the stresses induced between the compact and suppot by the shrinkage during sintering of the diamond grains. The barrier participates in the metallurgical bond, may be very thin and is formed of diamond and of tungsten carbide powder.
A thin nickel layer, obtained by coating the metal substrate, also participates in the intermetallic bond and forms an additional source of catalyst.
In practice, the metal substrate, if provided, will typically be of from some tenths of millimeter to some millimeters. A thickness of 0.25 mm is frequently satisfactory.
There is also provided a method of manufacturing such an abrasive product including the steps of: placing a layer of diamond powder with nickel and iron powder on a refractory metal substrate which is in contact with a tungsten carbide support; placing on said layer a disk or a fine layer of either silicon or nickel-titanium powder; placing a cell containing the so-built stack in a press at a temperature exceeding 1200.degree. C. under a pressure exceeding 45 kbars, the pressure and temperature being chosen so as to correspond to the zone of stability of diamond; and maintaining the temperature and pressure of the cell for a time sufficient for causing mutual plastic phase sintering of the diamond grains.
The invention will be better understood from the following description of particular embodiments, given by way of examples. The description refers to the accompanying drawings.